This invention relates to an integrated system for converting oxygenates, such as methanol or dimethyl ether (DME), to liquid hydrocarbons and ethene. In particular it provides a continuous process for producing hydrocarbon products by converting the oxygenate feedstock catalytically to an intermediate lower olefinic stream, separating the ethene and oligomerizing the remaining olefins to produce distillate and gasoline.
In order to provide an adequate supply of liquid hydrocarbons for use as synfuels or chemical feedstocks, various processes have been developed for converting coal and natural gas to gasoline, distillate and lubricants. A substantial body of technology has grown to provide oxygenated intermediates, especially methanol. Large scale plants can convert methanol or similar aliphatic oxygenates to liquid fuels, especially gasoline. However, the demand for heavier hydrocarbons has led to the development of processes for increasing yield of diesel fuel by a multi-stage technique.
Recent developments in zeolite catalysts and hydrocarbon conversion processes have created interest in utilizing olefinic feedstocks for producing C.sub.5.sup.+ gasoline, diesel fuel, etc. In addition to the basic work derived from ZSM-5 type zeolite catalysts, a number of discoveries have contributed to the development of a new industrial process, known as Mobil Olefins to Gasoline/Distillate ("MOGD"). This process has significance as a safe, environmentally acceptable technique for utilizing feedstocks that contain lower olefins, especially C.sub.2 -C.sub.5 alkenes. This process may supplant conventional alkylation units. In U.S. Pat. Nos. 3,960,978 and 4,021,502, Plank, Rosinski and Givens disclose conversion of C.sub.2 -C.sub.5 olefins, alone or in admixture with paraffinic components, into higher hydrocarbons over crystalline zeolites having controlled acidity. Garwood et al have also contributed improved processing techniques to the MOGD system, as in U.S. Pat. Nos. 4,150,062, 4,211,640 and 4,227,992. The above-identified disclosures are incorporated herein by reference.
Conversion of lower olefins, especially propene and butenes, over HZSM-5 is effective at moderately elevated temperatures and pressures. The conversion products are sought as liquid fuels, especially the C.sub.5.sup.+ aliphatic and aromatic hydrocarbons. Olefinic gasoline is produced in good yield by the MOGD process and may be recovered as a product or recycled to the oligomerization reactor system for further conversion to distillate-range products. Operating details for typical MOGD units are disclosed in U.S. Pat. Nos. 4,445,031, 4,456,779 (Owen et al) and 4,433,185 (Tabak), incorporated herein by reference.
In addition to their use as shape selective oligomerization catalysts, the medium pore ZSM-5 type catalysts are useful for converting methanol and other lower aliphatic alcohols or corresponding ethers to olefins. Particular interest has been directed to a catalytic process ("MTO") for converting low cost methanol to valuable hydrocarbons rich in ethene and C.sub.3.sup.+ alkenes. Various processes are described in U.S. Pat. Nos. 3,894,107 (Butter et al), 3,928,483 (Chang et al), 4,025,571 (Lago), 4,423,274 (Daviduk et al) and 4,547,616 (Avidan et al), incorporated herein by reference. It is known that the MTO process can be optimized by fluidized bed catalysis to produce a major fraction of C.sub.2 -C.sub.4 olefins economically.
It has been found that ethene production can be increased by supplementing a continuous fluidized bed unit with a smaller capacity fixed bed unit. In U.S. Pat. No. 4,506,106 (Hsia et al), incorporated herein by reference, there is disclosed a method for increasing the ethene production by employing a fixed bed reactor operated within a specified range of conditions. The ethene may be recovered in an interstage sorption unit as a valuable chemical feedstock, while the remaining olefins are oligomerized to predominantly C.sub.10 -C.sub.20 liquid hydrocarbons. Oligomerization process conditions tend to convert only a small portion of ethene as compared to C.sub.3.sup.+ olefins.